TEMPO (2,2,6,6-Tetramethylpiperidine-1-oxyl) is widely used as a radical trap, as a structural probe for biological systems in conjunction with electron spin resonance (ESR) or electron paramagnetic resonance (EPR) spectroscopy, as a reagent in organic synthesis, and as an initiator in polymer chemistry. Several derivatives of TEMPO are known. For example, TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) is a stable, cell-permeable nitroxide free radical that acts as a radical scavenger and nitric oxide spin trap. TEMPOL is known to impair mitochondrial function and induce apoptosis in a number of tumor cell lines through free radical dependent mechanisms. TEMPO and its derivatives may exert cyto-protective or cyto-toxic effects. Such dual behavior is common among antioxidants. Recently, TEMPO has been suggested for co-administration with other anti-cancer drugs.